High Efficiency Video Coding - Coding Efficiency

Coding Efficiency

The design of most video coding standards is primarily aimed at having the highest coding efficiency. Coding efficiency is the ability to encode video at the lowest possible bit rate while maintaining a certain level of video quality. There are two standard ways to measure the coding efficiency of a video coding standard, which are to use an objective metric, such as peak signal-to-noise ratio (PSNR), or to use subjective assessment of video quality. Subjective assessment of video quality is considered to be the most important way to measure a video coding standard since humans perceive video quality subjectively.

HEVC benefits from the use of larger Coding Tree Unit (CTU) sizes. This has been shown in PSNR tests with a HM-8.0 HEVC encoder where it was forced to use progressively smaller CTU sizes. For all test sequences when compared to a 64×64 CTU size it was shown that the HEVC bit rate increased by 2.2% when forced to use a 32×32 CTU size and increased by 11.0% when forced to use a 16×16 CTU size. In the Class A test sequences, where the resolution of the video was 2560×1600, when compared to a 64×64 CTU size it was shown that the HEVC bit rate increased by 5.7% when forced to use a 32×32 CTU size and increased by 28.2% when forced to use a 16×16 CTU size. The tests showed that large CTU sizes increase coding efficiency while also reducing decoding time.

The HEVC Main Profile (MP) has been compared in coding efficiency to H.264/MPEG-4 AVC High Profile (HP), MPEG-4 Advanced Simple Profile (ASP), H.263 High Latency Profile (HLP), and H.262/MPEG-2 Main Profile (MP). The video encoding was done for entertainment applications and twelve different bitrates were made for the nine video test sequences with a HM-8.0 HEVC encoder being used. Of the nine video test sequences five were at HD resolution while four were at WVGA (800×480) resolution. The bit rate reductions for HEVC were determined based on PSNR.

HEVC MP has also been compared to H.264/MPEG-4 AVC HP for subjective video quality. The video encoding was done for entertainment applications and four different bitrates were made for nine video test sequences with a HM-5.0 HEVC encoder being used. The subjective assessment was done at an earlier date than the PSNR comparison and so it used an earlier version of the HEVC encoder that had slightly lower performance. The bit rate reductions were determined based on subjective assessment using mean opinion score values. The overall subjective bitrate reduction for HEVC MP compared to H.264/MPEG-4 AVC HP was 49.3%.

École Polytechnique Fédérale de Lausanne (EPFL) did a study to evaluate the subjective video quality of HEVC at resolutions higher than HDTV. The study was done with three videos with resolutions of 3840×1744 at 24 fps, 3840×2048 at 30 fps, and 3840×2160 at 30 fps. The five second video sequences showed people on a street, traffic, and a scene from the open source computer animated movie Sintel. The video sequences were encoded at five different bitrates using the HM-6.1.1 HEVC encoder and the JM-18.3 H.264/MPEG-4 AVC encoder. The subjective bit rate reductions were determined based on subjective assessment using mean opinion score values. The study compared HEVC MP with H.264/MPEG-4 AVC HP and showed that for HEVC MP the average bitrate reduction based on PSNR was 44.4% while the average bitrate reduction based on subjective video quality was 66.5%.

In a HEVC performance comparison released in April 2013 the HEVC MP and Main 10 Profile (M10P) were compared to H.264/MPEG-4 AVC HP and High 10 Profile (H10P) using 3840×2160 video sequences. The video sequences were encoded using the HM-10.0 HEVC encoder and the JM-18.4 H.264/MPEG-4 AVC encoder. The average bit rate reduction based on PSNR was 45% for inter frame video.